This invention relates to an air/fuel ratio feedback control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to such air/fuel ratio feedback control system which is capable of achieving an accurate air/fuel ratio at an early time after resumption of closed loop or feedback control following open loop control, to thereby ensure high exhaust emission stability of the engine.
An air/fuel ratio feedback control system has already been proposed e.g., by the assignee of the present application, in which an air/fuel ratio control valve is controlled by means of an actuator such as a pulse motor in response to an output signal produced by an exhaust gas ingredient sensor such as an O.sub.2 sensor, provided in the exhaust system of an engine, so as to control the air/fuel ratio of an air/fuel mixture being supplied to the engine to a proper value to thereby achieve good engine driveability as well as exhaust emission characteristics.
According to this proposed air/fuel ratio feedback control system, the air/fuel ratio is sometimes not controlled to a proper value if the above air/fuel ratio feedback control based upon the output signal of the O.sub.2 sensor is carried out when the engine is in a particular operating condition other than partial load, such as wide-open-throttle, idle, deceleration and off-idle acceleration. Therefore, when the engine comes into such a particular operating condition, the feedback control system is released from its closed loop condition for feedback control of the air/fuel ratio and brought into open loop condition wherein the pulse motor position is moved to and held at a predetermined preset position appropriate for the particular engine operating condition concerned, thus obtaining a proper air/fuel ratio.
On the other hand, when the engine is in a partial load condition, the system is brought into the closed loop condition for feedback control of the air/fuel ratio. In this closed loop condition, proportional term control and integral term control are selectively carried out depending upon changes in the output signal (voltage) of the O.sub.2 sensor. More specifically, when the output voltage of the O.sub.2 sensor stays at a higher level or at a lower level with respect to a reference voltage, the position of the actuator is controlled with integral term correction in an accurate and stable manner, and when the O.sub.2 sensor output voltage changes from the higher level to the lower level or vice versa the actuator position is controlled with proportional term correction in a prompt and efficient manner.
Immediately after transition from open loop control to closed loop control, the pulse motor position must be controlled in immediate response to the output signal level of the O.sub.2 sensor to obtain a proper air/fuel ratio. However, there can be a difference in timing between the change of the output signal level of the O.sub.2 sensor from the higher level to the lower level or vice versa and the change from open loop mode to closed loop mode. In such an event, the pulse motor position can be largely deviated from its proper position upon entering the closed loop mode, at a rate corresponding to the above timing difference. This results in an improper air/fuel ratio obtained and accordingly unstable exhaust emission of the engine.